Magnetic particles deposited in the lungs can be detected from outside the body. Following inhalation or intratracheal instillation, retained particles are magnetized by a strong external magnetic field. After this field is turned off, the resulting remanent field produced by the aligned particles can be measured non-invasively by sensitive magnetometry. Data from these measurements contain information about two phenomena: First, the progressive decrease in the maximum attainable magnetic field each time an animal is magnetized indicates clearance of the particles from the respiratory tract. Secondly, immediately after the external magnetization, the remanent field decreases during one half hour to as little as 10% of its initial value. This phenomenon is called relaxation and is caused by gradual particle rotation following magnetization. Evidence from our experiments indicates that the movement of particle containing phagosomes and secondary lysosomes within macrophages is a major mechanism of relaxation. The general purposes of this proposal are to utilize a multidisciplinary approach to quantify the relative contributions of forces which cause relaxation, and to evaluate the magnetometric method as a possible tool for non-invasive assessment of macrophage function and lung injury. Our specific aims are: 1) To correlate changes in relaxation and clearance with changes in the anatomical location of magnetic dust over time, as determined by histological observations. 2) To better understand and interpret the mechanisms involved in relaxation: in vivo experiments. 3) To examine and interpret the behavior of magnetic particles in isolated pulmonary macrophages: in vitro experiments. 4) To compare magnetic and radioactive measurements of iron oxide clearance. 5) To correlate changes in magnetic relaxation and clearance with different types of lung injury.